polystyrene sheets on roof rafters.

[jw 1111]

Hi, when we first bought out house thirteen years ago ( an end house of four built in 1936 in north London. u.k.) I went up into the roof space one winter; and found the whole inside to be covered in a thin layer of snow. ( There was quite a high wind and it had blown the snow inside between the tile gaps ).

To remedy this problem i bought a roll of special roof plastic, (cannot remember the name, but it is made for this purpose and has an inbuilt lattice of nylon string for added strength ) which i laid up close to the tiles and stapled to each side of the wood rafters. This now keeps the snow out, but everything still gets covered in a thin layer of dust and grime.

We would like to further seal the roof to keep out this dust, but this does not have to be too elaborate, since the roof space is not a living space, but more of a storage space. Also i have laid rock wool insulation in the floor space so we do not need any extensive extra insulation properties on this project.

what i thought would be a relatively easy option is to get some of those building sheets of white polystrene type material and cut them into strips and nail them to the inside face of the roof rafters. they do not need to be all that strong ( but obviously must be stronger than ordinary polystyrene).

would anyone know the name of such a building material or any other building material that might suit better please? But being able to slice it into strips with a knife, would obviously be a big bonus for ease of application. thanks

[tbasc]

In the U.S., there are limits on the use of unprotected plastics of this kind because they develop toxic smoke in a fire.

In any case, I'm unclear about the source of the dust and about the ventilation of the attic space. Is the attic intended to be vented or unvented? Is the dust collecting on the tile side of the plastic sheeting or on some other surface? At what location do you think the dust is entering the attic? TB

[keith_765]

polystyrene).

application.

You should be able to buy one of the economical underlay felts, breather type, and staple it, not nail as this can cause tile to dislodge, to the under side of the rafters. Start at the ridge and work down to the eave, over lap horizontal by 6" or 150mm

[Goedjn]

Canvas. Give up on the existing roof, and build a tent in there covering the attic-access. Use mildew/fire resistant canvas. Use the existing roof-structure to keep the rain and heavy wind off, and depends on the tent for airflow controll. leave enough space for good airflow under the surface of the roof.

[SQLit]

polystyrene).

application.

Check out the last page first. I have used the JM material and it does do the job.

These are suggestions only as I do not understand living in a home where snow is in the attic. But then I do not live where there is snow very often, Phoenix.

[keith_765]

Ask any person who is in the roofing trade about spray on product. Don't breath, near imposable to remove broken tiles or slate in a repair. It's not even recommended by the roofing trade or roofing federation.

[Hugo Nebula]

On Wed, 21 Dec 2005 13:08:13 GMT, a particular chimpanzee named "jw

1111" randomly hit the keyboard and produced:

This will replace one problem with another. Your roof should be allowed to 'breathe' to prevent excess moisture building up in your rafters; any vapour impermeable material on the underside of the rafters should have ventilation over.

Would it not be better to tackle the problem at source and re-roof (thereby allowing you to put felt under the tiles)? I know a few late Victorian houses weren't felted, but I'm surprised that a 1930's house in London wasn't.

[keith_765]

If you read the breather felt application instruction they state that there should be a 10mm sag between the back of the lath in between rafters. this is to stop what they call ponding, condensation running down the underlay felt. Now if you read the tile manufactures instructions they say that there should be an air flow of 25mm between back of tile and underlay from eave to ridge. as of yet I've only used this method once. Lay breather underlay direct on to rafters, counter bittern on top of underlay and then lath across counter bitterns, with air vent strips on top of underlay at eave, with dry ridge vent system at ridge.

I wonder what they will dream up next.

Keith roof tiler 52 years

[keith_765]

By the way felt underlay wasn't generally used on roofs until the early 50s.

Did you also know that there isn't a plastic fascia that's is not designed for load bearing to support roof tiles, they all recommend a solid timber support.

[jw 1111]

Many thanks to all. Following the suggestion of a friend, I rang the Barnet council building department ( I live in north london u.k.). They said to buy 2 inch (50 mm) thick foam polyurathane 8 ft x 4ft sheets and slice it up with a knife and push it up between the roof joists up to the plastic i have already put up there.

They did not know the exact manufacturer of these sheets or where best to buy them from. Would anyone local to North London be able to give any further pointers on where to get these sheets please? Thanks.

[keith_765]

Builders merchant. The material is called Xtherm pronounced extra therm

[mrcheerful]

The only proper repair is to strip and relay the roof.

mrcheerful

[jw 1111]

Considering the extra expense; why is that then ?

>

[mrcheerful]

It will be impossible to achieve a perfect job by any internally applied stuff, also it will give condensation and rot problems.

A strip and relay costs very little in materials and since this is a diy newsgroup the labour should be negligible too.

mrcheerful

[keith_765]

Correct!. To try to lay underlay felt inside would in fact stop the dust. The question was "How to stop dust from entering the ceiling" To keep the weather elements out like snow blowing in and dust, then the underlay must be laid on top of the rafters. This eliminates try to lap the underlay into the ridge tree, hip rafters and over the load bearing perlins and most important completely covering the void at the eaves level over the fascia into the gutter. That's if you want to spend about £5000 depending on the condition of the tiles or slates or diy for about £2000. Requirement, A load bearing scaffolding, ladder, waste skip, new felt,+ felt nails, new lath's + lath nails, sand & cement, make up any shortage on tiles/ slates + fixing nails and its not a single handed job and you've got to keep it watertight at all times . There's a few thousand of tiles / slates on a roof of a 1930s house. Still fancy having ago.

[jw 1111]

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So would you do it for me for £5000 ? You said in an earlier post that you are a roof tiler for 52 years.

But surely the 50mm insulation board can just as easily be put up from the inside? Also even if its not a perfect job at keeping out dust the underlay could very carefully be laid up to ' the ridge tree, hip rafters and over the load bearing perlins etc etc etc' as you mentioned; the difference between a tiny bit of dust (especially it would be minimal if the insulation board is laid in as well) would well be worth it, if i am saving about £4,800 ???

especially since you advised in your post to this group of 25 December that:

"You should be able to buy one of the economical underlay felts, breather type, and staple it, not nail as this can cause tile to dislodge, to the under side of the rafters. Start at the ridge and work down to the eave, over lap horizontal by 6" or 150mm"

[mrcheerful]

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With 52 years experience he is a bit old now! The point is you can do a bodge up which will never be perfect, or you can do it properly, the second will give you a perfect, long lasting job, the former an imperfect, leaking and rot promoting bodge up, which will reduce the dust and hide the rot, till the next owner has to fix it or a survey rejects it.

mrcheerful

[jw 1111]

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But surely insulation board can just as well be installed from the inside as the outside?

[Goedjn]

It can, yes, but unless you solve the problem of water coming though the tile, doing so will trap water against the structure of the roof, between the foam and the tile. This will destroy the roof.

If you are not going to fix the water-shedding layer, you are far better off letting the water and whatever else is coming through the tile come all the way through, away from the roof, before catching it and leading it out whatever outlet you can find.

I suggest getting a big attic fan, and blowing air into the attic through a gable-end, and letting the air blow out through the tile. If you keep the attic under positive pressure, and put a filter at the inlet, you'll keep dust down and reduce water infiltration, and it will take few years for the excess energy cost to eat your "savings" in not fixing the roof.

[keith_765]

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Woe Woe!! Cancel the scaffolding. After reading all the advise given , you have answered your own question in your first mail. You've put some kind of sheeting up to the back of the tile lath and you say this as stopped the drifting snow, right that's cured that problem. So now you want to put some type of white polystyrene sheet cut them to fit in between the rafters, white polystyrene is a fire hazard, better if you use Xtherm 25mm /1" thick or even plaster board but what ever you use you must tape the joints.